1. Field of the Invention
The present invention relates to an on-vehicle radar device for detecting an object in front by radio waves, and more particularly to an on-vehicle radar device which sets a threshold of reflection wave intensity from an object for each area, and detects an object based on the threshold being set.
2. Description of the Related Art
On-vehicle radar devices installed on the front of a vehicle to measure a distance to an object from the front of the vehicle and a relative velocity of the vehicle and the object have been proposed. By measuring the distance up to the object and the relative velocity, an alarm device for warning of an impending collision with an object in advance can be activated, or a brake can be activated to prevent collision with an object.
As a conventional on-vehicle radar device, for example, phase mono-pulse type on-vehicle radar device is proposed. The phase mono-pulse type on-vehicle radar device transmits a radio wave beam in a millimeter wave area, receives the wave reflected by an object using a plurality of receive antennas, and determines a direction of the object based on the phase difference of the receive waves.
FIG. 1 shows an example of receiving a reflection wave in the conventional phase mono-pulse type on-vehicle radar device. In FIG. 1, θ is an angle to indicate the direction of an object when the front direction of the vehicle is θ°. The distance D is a space between two receive antennas, and the phase difference φ is a phase difference of the two receive waves. The two receive antennas 14a and 14b receive the waves reflected by the object existing in an angle θ direction. The angle θ which indicates the direction of the object, which reflects the waves to be received by the two receive antennas 14a and 14b, is determined by the following Expression.θ=sin−1(λφ/2πD)   [Expression 1]
Here λ is a wavelength.
In such an on-vehicle radar device, a predetermined value is set for the threshold of the reflection wave intensity for detecting an object for all the directions. If a reflection wave having an intensity exceeding this threshold is detected in a specific direction, it is judged that an object exists in this direction.
On the other hand, Japanese Patent Application Laid-Open No. 2005-009913, for example, discloses a technology to make detection of an object easier by providing separate thresholds to the center area of the detection range of the reflection wave and the edge area of the detection range according to the intensity distribution of the transmission wave.
According to this technology, the threshold of the reflection wave detection is increased at around 0° which is the center area of the object detection range, so as to block noise and make detection of the object easier. Whereas near the edge of the detection range, the threshold of the reflection wave detection is decreased so as to receive the reflection wave easily. This kind of setting the threshold corresponds to the characteristic of the antenna where the intensity of the transmission wave becomes maximum amounts at the center, and decreases moving toward the edge.
Similar technologies are also disclosed in Japanese Patent Application Laid-Open No. 2003-267084, Japanese Patent Application Laid-Open No. Hei 9-506698 (corresponding U.S. Pat. No. 5,633,642) and Japanese Patent Publication Laid-Open No. Sho 64-65475, for example.
With such detection methods, however, the vehicle in front existing at the center area of the detection range is hard to be detected. Particularly in an active cruise control system (ACC) that detects a vehicle in front which is traveling on a same lane by radar and adjusting the velocity automatically according to the velocity of this vehicle in front, the vehicle in front that is supposed to be followed up may be missed in detection.
That is, if a threshold is set according to the intensity of the transmission wave as the case of the prior art, the threshold of the intensity of the receive wave is low at the edge area of the detection range, and the threshold of the intensity of the receive wave at the center is high. Since this means to increase the threshold in the center area, where the probability of detecting a vehicle in front is high, it causes a detection omission of a vehicle in front, and makes follow up of the vehicle in front difficult in an active cruise control system where detection of a vehicle in front to be followed up is critical.
Also in such a traveling situation as a curve, a vehicle traveling in front on the same lane may be miss-detected as a vehicle traveling in front on a different lane, and a detection error may occur.
A radar device which is mounted on a vehicle is used not only for an active cruise control system (ACC), but also for pre-crash safe systems (PCS) for detecting an obstacle, such as a guard rail, on-coming vehicles, and a vehicle traveling next to it in a same direction, in advance, and avoiding it.
Which ACC or PCS are more critical changes depending on the traveling situation of the vehicle, and the information that should be supplied depends on the system. However a method of setting thresholds for supplying information according to the change of the situation of the vehicle has not been proposed.